On Monday, the Food and Drug Administration said the key nutrient in Impossible Foods' "bleeding" veggie burger recipe is safe to eat, or in official parlance, "generally recognized as safe" (GRAS). That nutrient is heme, an iron-rich ingredient that's found naturally in the bodies of all living things. It's also what gives the Impossible Burger its deep, blood-red color when cooked.

The FDA nod is a big win for Impossible Foods, the San Francisco-based startup behind the burger.

After receiving funding from Bill Gates, inking deals with major fast-food chains like White Castle, and even making its burgers available on Air New Zealand flights, the company had literally been flying high. But it hit a snag last summer when, after voluntarily sending data on behalf of its burger to the FDA, the agency responded with a red flag: they said the paperwork wasn't sufficient to "establish the safety" of heme.

Taken aback by the FDA's response, Impossible Foods came back to the agency with more research detailing heme's safety. On Monday, the agency essentially reversed its original statement and concluded that heme — and the Impossible Burger — were safe to consume.

Heme, the essential nutrient you've never heard of

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Although the scientists at Impossible Foods had already done extensive testing on heme, they did not get the official nod from the FDA until Monday.

Although today's veggie burgers can be described with a handful of delicious-sounding adjectives, "meaty" wasn't really one of them until Impossible Foods came along. The meat-like flavor in the Impossible Burger can largely be attributed to heme, an essential nutrient in many proteins that is found in just about every living thing on Earth.

In our bodies, heme is found tucked inside of a molecule in our blood called hemoglobin. Heme is also found in smaller concentrations in some plants including soy, which is where the Impossible Burger's heme is from. By genetically modifying yeast, Impossible Foods turns the organisms into tiny heme factories that churn out enough of the ingredient to give the burgers their meaty flavor.

Brown said the FDA's green light underlines what he and the rest of the Impossible Foods team already knew — that heme and the Impossible Burger are safe to eat.

"We would have been kicking ourself in the foot if we hadn't already done the research and proven that this was safe," Brown said. "But it's great news."